After thorax surgery or during pneumothorax treatment, air leaks into a thoracic cavity of a patient. Such leaking of air into the thoracic cavity of the patient is generally termed air leak. In order to discharge to the exterior of the patient's body air that has leaked into his thoracic cavity or bodily fluid such as blood or the like that has accumulated in his thoracic cavity, procedures are performed to connect a thoracic cavity drain bag to a drain tube that is indwelt in the thoracic cavity of the patient after thoracic surgery or during pneumothorax treatment, and also to connect an suction equipment to this thoracic cavity drain bag so as to suck air that has leaked into the thoracic cavity and/or bodily fluid such as blood or the like that has accumulated in the thoracic cavity.
A drain chamber that traps liquid such as bodily fluid or the like that has been sucked from the thoracic cavity and a water seal chamber that contains a liquid such as water and thereby prevents reverse flow of external air toward the thoracic cavity are formed in the thoracic cavity drain bag. Air consequent upon air leak that has been sucked from the thoracic cavity passes through the liquid in the water seal chamber and is led to the exterior. When air passes through the liquid in the water seal chamber, air bubbles are generated. Accordingly, by a health care attendant observing air bubbles that are generated in the water seal chamber, it is possible to check upon the frequency and the state of occurrence of air leak and thus to ascertain the state of recovery of the patient, and thereby it is possible to decide about the timing for changing the drainage of the thoracic cavity from suction to simple gravity drainage, and the timing for removal of the drain tube. However it is difficult to observe the generation of air bubbles over time, since the frequency of generation of air bubbles in the water seal chamber decreases along with the progression of healing of the air leak.
Thus, as a detection device that detects air bubbles in the water seal chamber, a detection device has been proposed (refer to Patent Document #1) that detects air bubbles in the water seal chamber with an optical sensor, instead of by employing visual inspection. Moreover, a device is per se known (refer to Patent Document #2) that calculates the total amount of sucked air and liquid from the operational state of the vacuum pump, and that calculates the amount of sucked air by subtracting the amount of liquid accumulated in the drain chamber of the thoracic cavity drain device from this total sucked amount. Apart from the above, Patent Document #3 may be considered as being a document having some relevance to the present invention.